fix(shellterm): resolve session workspace path for terminals opened f…

[anmol0b]

What

Adds session-scoped workspace resolution to OpenShellTerminal, so a standalone terminal opened while viewing an agent session starts in that session's worktree instead of always defaulting to the project's main root.

Why

Fixes #3061. A terminal opened from a session view looked co-located with the active agent, but ran against a different checkout/branch — the project root, not the session's worktree — making it easy to inspect, build, or edit the wrong tree.

How

• Added SessionWorkspaceLocator and wired a concrete implementation through daemon.go/shellterm_wiring.go, so shellterm.Service can resolve a session id to its durable workspace path (SessionRecord.Metadata.WorkspacePath) rather than only ever consulting ProjectRootLocator.
• resolveShellTerminalWorkingDir now checks the session locator first when a sessionId is present, falling back to the project root if the session exists but hasn't resolved a workspace yet, and to the existing project-root/data-dir fallbacks when no session id is sent at all.
• No controller, DTO, or frontend changes were needed — sessionId was already threaded end-to-end from useShellTerminals.ts through OpenShellTerminalRequest/OpenShellTerminalInput; the gap was purely that the service layer never used it once it arrived.
• Both the terminal-tab + action and the Ctrl+Shift+` shortcut share the same useOpenShellTerminal mutation, so both pick up the fix identically without separate handling.

Testing

Backend (backend/):
New/updated backend tests: TestOpenShellTerminalStartsInSessionWorkspaceNotProjectRoot, TestOpenShellTerminalFallsBackToProjectRootWhenSessionHasNoWorkspaceYet, TestOpenShellTerminalReturnsNotFoundForUnknownSession, TestOpenShellTerminalWithoutSessionIgnoresSessionLocator, and TestOpenShellTerminalScopesToSession (updated to use a populated session locator).

Frontend (frontend/):
New test: "scopes the terminal to the session in scope", alongside the existing "scopes the terminal to the project in scope".

Checklist

• Branched from main (or continuing an existing PR branch)
• One focused change; links the related issue when applicable
• Follows AGENTS.md conventions and PR hygiene
• Tests added/updated for user-visible behavior where it makes sense
• Relevant CI checks pass for the area touched (go, frontend, etc.)

[somewherelostt]

Thanks for contributing to Agent Orchestrator.

This PR is being picked up by the current external contributor on-call pair:

• @illegalcall
• @Pulkit7070

If someone is already working on this, please continue as usual.
The on-call pair is added for visibility, tracking, and support, not to take over the work.
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